High and low temperature tensile strain at break testing measures the ductility and fracture behavior of plastic materials under uniaxial tensile loading across extreme service temperatures. Conducted in accordance with ISO 527 and ASTM D638 using environmental chambers and precision extensometry, this test is critical for evaluating material embrittlement at low temperatures, softening at elevated temperatures, and long-term structural reliability. This article covers the technical principles, specimen requirements, test methods, applications, and frequently asked questions related to plastic tensile failure strain testing at high and low temperatures.
Technical Overview
Tensile strain at break, also known as elongation at break or failure strain, quantifies the maximum engineering strain a plastic specimen sustains before fracture under uniaxial tensile loading. It is calculated as the percentage increase in gauge length at the moment of break relative to the initial gauge length, and it is one of the most sensitive indicators of material ductility and toughness.
Because plastics are viscoelastic materials, their stress-strain response is strongly dependent on temperature and loading rate. At sub-ambient temperatures, polymer chain mobility is restricted, many thermoplastics approach their glass transition or secondary relaxation regions, and failure strain drops sharply as the fracture mode shifts from ductile to brittle. At elevated temperatures, molecular mobility increases, ductility generally rises while strength declines, and thermal degradation may occur beyond certain limits. Testing strain at break across a defined temperature envelope therefore reveals the ductile-to-brittle transition window and defines the safe operating range of a material.
High and low temperature tensile failure strain testing is performed on a universal testing machine equipped with an environmental chamber, typically covering −70 °C to +300 °C and extendable to lower temperatures with liquid nitrogen cooling. Strain can be captured with contact extensometers or non-contact video extensometry. The governing test methods include ISO 527-1 (general principles), ISO 527-2 (moulding and extrusion plastics), ISO 527-3 (films and sheets), ASTM D638, ASTM D882 for thin films, and the corresponding GB/T 1040 series for the Chinese market. Tests are commonly carried out by accredited third-party laboratories or in-house materials laboratories.
Testing Samples
Moulding and Extrusion Plastics
Rigid and semi-rigid thermoplastics and thermosets are tested using multipurpose dumbbell specimens produced per ISO 3167 and tested as ISO 527-2 type 1A or 1B specimens, or as ASTM D638 type I through type V specimens depending on thickness. Typical materials include PE, PP, ABS, PC, PA6, PA66, POM, PBT, PET, PVC, PMMA, PSU, PPS, PEEK, and their blends and alloys, with preferred specimen thicknesses of 1 mm to 4 mm and up to 14 mm for thicker sections.
Plastic Films and Sheets
Flexible packaging films and thin sheets below 1 mm are tested using strip or dumbbell specimens per ISO 527-3, with very thin films often evaluated per ASTM D882 using long-travel strain measurement. Common materials include PE, PP, PA, PET, PLA, and multilayer laminates, where failure strain at refrigerated and frozen temperatures is a key cold-chain performance metric.
Filled and Reinforced Plastics
Glass-fiber, carbon-fiber, and mineral-filled compounds exhibit anisotropic behavior driven by filler orientation during moulding. Specimens are typically cut along and perpendicular to the flow or fiber direction to characterize directional ductility. Reinforced grades show higher modulus but substantially lower strain at break, and brittle fracture behavior is more pronounced at low temperatures.
Components, Profiles, and Additively Manufactured Parts
Specimens can be machined from finished parts, extruded profiles, or injection-moulded components per ISO 2818 when original mouldings are unavailable. Additively manufactured coupons from FDM, SLA, or SLS processes are tested in multiple build orientations to capture layer adhesion effects, which become especially critical at low service temperatures.
Testing Items
The primary measured parameter is the tensile strain at break at each specified temperature, supported by complementary tensile properties and statistical reporting:
- Tensile strain at break, or elongation at break, expressed as a percentage at each specified temperature
- Nominal strain at break based on grip separation and extensometer-based strain at break using gauge length
- Tensile stress at break and tensile strength at temperature, expressed in MPa
- Tensile modulus at temperature, measured at 1 mm/min within the 0.05 % to 0.25 % strain range per ISO 527-1 when required
- Failure mode classification as ductile, semi-ductile, or brittle fracture
- Statistical summary including mean value, standard deviation, and coefficient of variation for each condition
Common temperature set points include −70 °C, −55 °C, −40 °C, −30 °C, −20 °C, 0 °C, 23 °C as the reference condition, 40 °C, 60 °C, 80 °C, 100 °C, 125 °C, 150 °C, 180 °C, 200 °C, and 250 °C. Custom temperature matrices can be designed around a material's glass transition temperature or a specific application envelope to map the ductile-to-brittle transition.
Testing Methods
Specimen Preparation and Conditioning
Specimens are injection moulded or machined to the applicable geometry, inspected for surface defects and notch-like imperfections, and conditioned in a standard atmosphere of 23 °C ± 1 °C and 50 % ± 5 % relative humidity per ISO 291 or ASTM D618, typically for at least 24 hours. Hygroscopic materials such as polyamides are tested in defined moisture states, for example dry-as-moulded or equilibrated to 50 % relative humidity, because moisture content significantly affects failure strain. Specimen width and thickness are measured at the narrow section to an accuracy of 0.02 mm before testing.
Thermal Conditioning and Chamber Control
The environmental chamber is pre-equilibrated at the set point with a calibrated thermocouple positioned adjacent to the specimen, and temperature is typically controlled within ±2 °C. Standard 4 mm thick specimens are thermally soaked for 5 to 10 minutes before loading, with extended soak times for thick sections or low-thermal-conductivity materials. Cooling is achieved with liquid nitrogen or carbon dioxide systems and heating with resistance elements and forced-air circulation. High and low temperature rated grips are used, and a small preload is applied at temperature to compensate for thermal contraction and remove slack from the load path.
Strain Measurement Techniques
The reference gauge length is typically 50 mm. Contact clip-on extensometers with high or low temperature variants provide direct strain measurement, while non-contact video extensometers tracking gauge marks through the chamber window are preferred at temperature extremes and for brittle failures where knife-edge contact could influence fracture. Nominal strain calculated from grip separation or crosshead displacement includes system compliance and seating effects, which is acceptable for brittle materials failing below roughly 5 % strain but overestimates strain for ductile materials. Long-travel measurement is applied where failure strain exceeds 100 %.
Test Execution and Crosshead Speeds
ISO 527-1 specifies preferred test speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min. Modulus is commonly determined at 1 mm/min, while strain at break for type 1A and 1B specimens is typically measured at 50 mm/min unless the material standard specifies otherwise. ASTM D638 specifies speeds according to specimen type, for example 50 mm/min for type I specimens. At least five valid specimens are tested at each temperature, and breaks occurring at the grips or outside the gauge region are discarded and retested, as required by ISO 527-1.
Data Reduction and Reporting
Tensile strain at break is computed as the change in gauge length between the initial state and break, divided by the initial gauge length, expressed as a percentage. The test report documents material identification and lot, conditioning history, specimen type and dimensions, test temperature with tolerance, crosshead speed, gauge length, strain measurement method, individual and mean results with scatter, and failure mode classification. When a temperature matrix is tested, results are plotted as strain at break versus temperature to identify the ductile-to-brittle transition region using criteria such as a defined minimum failure strain threshold.
Applications
High and low temperature tensile failure strain data supports design validation, material selection, qualification, and quality control across many industries:
- Automotive and electric vehicles: validation of under-hood plastics, battery housings, connectors, cooling system components, and interior parts from −40 °C cold-start conditions up to +150 °C engine-bay temperatures
- Aerospace: qualification of interior panels, ducting, clips, and polymer matrix materials from −55 °C high-altitude service temperatures to elevated thermal exposure
- Electronics and electrical: assessment of connectors, sockets, enclosures, and surface-mount components that must retain ductility after reflow soldering and during low-temperature transport
- Packaging and cold chain: verification of film and laminate toughness at refrigerated and frozen storage temperatures down to −40 °C and after high-temperature filling or sterilization
- Pipes, fittings, and infrastructure: evaluation of PE, PVC, and PP pressure pipe materials for low-temperature installation toughness and elevated-temperature service life
- Medical devices: characterization of sterilizable polymers and cryogenic storage consumables across sterilization and storage temperature extremes
- Research and development: generation of temperature-dependent material cards for finite element simulation, competitive benchmarking, and incoming material batch release
FAQ
What is the difference between tensile strain at break and elongation at break?
The two terms describe the same quantity: the relative extension of the specimen at fracture. Strain at break is the dimensionless ratio expressed as a percentage, while elongation at break is the conventional term used in many older standards. The more important distinction is between nominal strain, based on grip separation, and extensometer-based strain, based on gauge length, with the latter being the preferred and more accurate method.
What temperature range can be tested?
Standard environmental chambers cover approximately −70 °C to +300 °C. Liquid nitrogen cooling can extend the lower limit to approximately −150 °C or below for special applications. The practical range is also constrained by grip materials, extensometer hardware, and the thermal stability of the plastic itself.
Which specimens are required, and how many per condition?
ISO 527-2 type 1A or 1B dumbbell specimens or ASTM D638 type I to V specimens are used for moulded plastics, while ISO 527-3 strips or ASTM D882 specimens apply to films. A minimum of five valid specimens per temperature is required by ISO 527-1, and additional specimens are recommended for filled or highly variable materials.
How is strain measured at extreme temperatures?
Non-contact video extensometry through the chamber viewing window is the preferred method because it avoids mechanical influence on the specimen and works at any temperature. Contact extensometers designed for high or low temperature operation are also used, and nominal strain from crosshead displacement serves as a fallback with known compliance limitations.
Why does failure strain decrease at low temperatures?
Reduced molecular mobility increases yield stress and suppresses plastic deformation mechanisms such as shear yielding and crazing. As the material approaches or drops below its glass transition or secondary relaxations, fracture becomes brittle, and failure strain can fall from several hundred percent to just a few percent.
What acceptance criteria are typical for strain at break?
Criteria are application specific and defined by product standards, end-user specifications, or design requirements. For example, a ductile design may require strain at break to remain above a defined minimum, such as 20 %, at the lowest service temperature, while brittle behavior is often flagged when failure strain falls below approximately 5 %. Comparisons should always reference the same standard, specimen type, speed, and strain measurement method.